Keplerian viewfinders comprising a positive objective lens and a positive ocular lens are favored for use as viewfinders on high-quality lens-shutter cameras. In such cameras, keplerian viewfinders allow fields of view, partitioned fields of view and various types of displays to be clearly viewed by arranging the field-of-view frame and the reticle close to the focal point of the objective lens.
The entrance pupil of a keplerian viewfinder is inside the viewfinder itself or on the object side of the viewfinder. Thus, such a viewfinder can be used as a "zoom" viewfinder (in which the viewfinder magnification can be continuously changed over a range) or a "wide-angle" viewfinder while avoiding a need to greatly increase the diameter of the objective lens.
Examples of conventional keplerian viewfinders are disclosed in Japanese Kokai (laid-open) patent document no. 233420 (1991) and Kokai patent document no. 242377 (1994). Such viewfinders comprise, in order from the object side, a first lens group having a negative refractive power and a second lens group having a positive refractive power. A third lens group having a negative refractive power serves as the objective lens.
In recent years, the evolution of lens-shutter cameras has tended toward increasingly smaller camera size. Increasingly smaller cameras require increasingly smaller viewfinders without decreasing the size, brightness, and viewing ease of the image produced by the viewfinder. Also, in recent years, the zoom ratios of lens-shutter cameras have increased. This trend has generated a need to increase the zoom ratio of the viewfinder used with such cameras without significantly increasing the size of the viewfinder.
A compact viewfinder is disclosed in Kokai patent document no. 233420 (1991), wherein each lens group consists of only one lens, and the viewfinder exhibits favorable correction of aberrations. Unfortunately, however, the viewfinder magnification at the wide-angle end is about 0.4, and the zoom ratio is no greater than 2, which is not regarded as sufficiently large for current needs. The viewfinder disclosed in Kokai patent document no. 242377 (1994) has a zoom ratio of 2 or greater, but the magnification on the wide-angle end is about 0.3 and each lens group consists of more than one lens. Such configurations are too costly and bulky for many applications.